The existence of copolymers of carbon monoxide with olefins, especially ethylene, is well known in the prior art. The copolymerization of ethylene and carbon monoxide using free radical initiators has been described by Brubaker et al. (U.S. Pat. No. 2,495,286) Furthermore, copolymerization of ethylene and carbon monoxide initiated by gamma-irradiation has been claimed by Columbo et al. in Journal of Polymer Science, Part Al, Vol. 4 (1966) p. 29. These methods produce polymers with carbon monoxide contents ranging from less than 1% to as much as 50%.
Linear alternating copolymers and terpolymers of carbon monoxide and .alpha.-olefins, especially ethylene (ECO), having the general formula [--(CO) (ol)-].sub.n, where ol are .alpha.-olefin monomers, have been prepared using transition metal complexes as described by Nozaki (U.S. Pat. No. 3,694,412), Fenton (U.S. Pat. No. 4,076,911), Lai and Sen (Organometallics, Vol. 3 (1984) p. 866) and Drent (U.S. Pat. Nos. 4,818,810 and 4,835,250).
Reduction of .alpha.-olefin-carbon monoxide copolymers produces polyalcohols. Polyalcohols can be cast or extruded into films and used as adhesives or coatings for paper. Polyalcohols also show a a high degree of barrier properties. This is of advantage in packaging for food, drug and cosmetics, which usually undergo oxidative degradations when exposed to air.
Polyalcohols such an ethylene-vinyl alcohol (EVOH) are generally produced by hydrolysis of ethylene-vinyl acetate copolymers (EVA), not reduction of .alpha.-olefin copolymers. Polyalcohols produced from .alpha.-olefin-carbon monoxide copolymers have a number of advantages over EVOH produced from the hydrolysis of EVA. These are:
1) carbon monoxide is a less expensive feedstock than vinyl acetate; PA0 2) hydrolysis of EVA to EVOH results in the formation of acetate salts as byproducts (actate salt byproduct are not formed when .alpha.-olefin CO copolymers are reduced); PA0 3) Reduction of linear, alternating .alpha.-olefin-carbon monoxide copolymers produces polyalcohols of highly regular structures (EVOH produced by the hydrolysis of EVA does not produce highly regular structures); PA0 4) polyalcohols made by the reduction of alternating ECO copolymers have a greater density of hydroxyl groups (I) than EVOH made from EVA (II):
I Fig. A. (--CH.sub.2 --CH.sub.2 --CH(OH)--).sub.n PA1 II. Fig. B. (--CH.sub.2 --CH.sub.2 --CH.sub.2 --CH(OH)--).sub.n.
Furthermore, polyalcohols represented by formula I from alternating ECO copolymers have high molecular purity and higher stability than those represented by formula II due to the structural inability of polyalcohols represented by formula I to dehydrate to conjugated dienes, as those represented by formula II are known to do.
Two principal methods have been used for the reduction of these carbon monoxide copolymers to polyalcohols. One is hydrogenation using a transition-metal catalyst. Brubaker et al. (Journal of the American Chemical Society, Vol. 74 (1952) p. 1509) and van Broekhoven (U.S. Pat. No. 4,929,701) describe the use of copper chromite or nickel catalysts to reduce polyketones to polyalcohols. However, the high temperatures and pressures used in this process can result in dehydration of CO-rich copolymers to 2,5-tetrahydrofurandiyl functionalities, which have inferior barrier properties. Furthermore, Wong et al. in European Patent Application 0 322 976 (1988) teach the hydrogenation of polyketones using nickel salts reacted with borohydrides as catalysts.
It is, of course, well known that borohydride salts can be used to reduce ketones to alcohols under mild conditions. There are a number of examples in the prior art of the reduction of polyketones like ECO by borohydride salts. JP 82-015907-B4 uses hydrogenated boron sodium, presumably sodium borohydride. Morashima et al. (European Polymer Journal, Vol. 9 (1973) p. 669) also used sodium borohydride in methanol. Wong et al. (European Patent Application 0 322 976) and Wong (U.S. Pat. No. 4,868,254) prefer reductions of .alpha.-olefin-carbon monoxide copolymers by sodium borohydride in hexafluoroisopropanol or m-cresol.
All these examples imply or teach that polyketone reductions by borohydride require the use of alcoholic media so that the starting polyketone and/or the product polyalcohol may dissolve. There are a number of disadvantages, however, in carrying out these reactions in alcohols. Common borohydride salts such as sodium borohydride react with alcohols to form tetraalkoxy- or tetraaryloxyborates. This type of decomposition is particularly rapid in methanol. Ethanol or isopropanol react more slowly with sodium borohydride, but the solubility of the borohydride in these solvents is low (respectively 4 and 0.25 grams per 100 ml of solvent), resulting in large amounts of effluent after the reduction. Other solvents, such as hexafluoroisopropanol or m-cresol, are toxic and expensive.
In light of the deficiencies described, the need for an improved process for the reduction of .alpha.-olefin-carbon monoxide copolymers and terpolymers which uses an inexpensive, non-toxic solvent in which the reducing agent is both highly soluble and stable is deemed readily apparent.